In Our Skies: Sunspots mission keeps an eye on the sun

One of our nearby astronomical research institutions has been in the news lately, although – despite the various conspiracy theories that have been floating around – for entirely non-astronomical reasons.

During all the recent hoopla it is perhaps worthwhile to keep in mind the Sunspot Solar Observatory’s raison d’etre: along with several other solar observatories around the world, as well as several orbiting spacecrafts, Sunspot’s mission is to keep an eye on the nearest of all stars, the sun.

It is a fair statement that, without the sun, life here on Earth would be impossible; indeed, life never would have gotten started on Earth in the first place. Either directly or indirectly, it is the source of all the energy that life, including us, needs to survive, and furthermore events that take place on the sun can have dramatic effects here on Earth.

It thus behooves us to do everything we can to understand just what is going on at our parent star.

Certainly, one of the most important issues concerning the goings-on within the sun is how it generates its energy in the first place. This remained a mystery for quite some time, with various mechanisms being proposed but all turning out to be unsatisfactory and unable to explain all the observed phenomena.

Finally, in 1920 the British astrophysicist Arthur Eddington proposed that the sun generates its energy via the process we now call nuclear fusion, and the details of how this process operates were worked out over the subsequent couple of decades.

Nuclear fusion requires an environment of extreme temperatures and pressures for it to proceed, but in the interior of the sun, where the overlying enormous amounts of matter create those conditions – for example, a temperature of some 25 million degrees Fahrenheit – fusion proceeds apace.

In simple terms, within the process of fusion the nuclei of four hydrogen atoms combine to form the nucleus of a helium atom. In reality, the process is more complicated than this, involving a series of nuclear reactions collectively termed the proton-proton cycle.

The resulting helium nucleus is some 0.7 percent less massive than the combined mass of the four hydrogen nuclei that went into it; this missing mass has been converted into energy via Albert Einstein’s famous equation E = mc2. While 0.7 percent of an atom might not seem like very much, with some 600 million tons of hydrogen being fused into helium every second, that ends up being a lot of energy.

The energy generated by nuclear fusion is primarily in the form of a high-frequency type of light called gamma-rays. Because the matter deep within a star is so dense, any given gamma-ray can only travel so far – typically a tiny fraction of a millimeter – before it is absorbed by an atom.

That atom then re-emits that gamma-ray, but in a totally random direction – where it is then re-absorbed by another atom, re-emitted again in a random direction, re-absorbed by yet another atom, and so on.

The gamma-ray’s path outward is an example of a famous mathematical problem known as the random walk, and the result is that any given gamma-ray can take tens of thousands – even a couple of hundred thousand or more – years to travel 70 percent of the way to the sun’s surface (300,000 miles).

The energy is then transported the rest of the way to the sun’s surface – or photosphere – via the process known as convection. This is the same process via which boiling water transfers heat from the bottom of a saucepan to the top – except that the material in the sun’s convection zone is not a liquid but rather a plasma, i.e., an ionized gas.

High-resolution images of the photosphere – for example, those taken by the Dunn Telescope atop Sacramento Peak – show these convection cells constantly forming and dissipating. Meanwhile, at the photosphere the energy is converted into light that is radiated into space, and 81Ž2 minutes later a small fraction of that sunlight arrives at Earth.

The sun is rotating, and that rotation, combined with the plasma in the sun’s convection zone, generates a strong magnetic field. When the concentrated force lines of this magnetic field intersect the photosphere, they create the dark regions we call sunspots, which appear dark only because they are some 2,000 degrees cooler than the surrounding photosphere although, at approximately 7,000 degrees F, still very hot by our standards.

Through processes that are still not entirely understood, the sun’s magnetic field also generates solar flares, induces changes in the sun’s outer atmosphere, or corona, creates the plasma releases known as coronal mass ejections (CMEs), and effects changes in the solar wind, the stream of charged particles constantly blowing off the sun’s corona.

These various phenomena can sometimes drastically affect the goings-on here on Earth.

Fortunately, the Earth has its own magnetic field, which helps in deflecting some of this activity away from us, at most producing the light displays we know as an aurora. However, strong solar flares and CMEs, if they should strike Earth, can still wreak havoc with our power grids and with the electronics aboard orbiting satellites.

In a society as dependent upon modern electronics technology as ours, the advance warning provided by solar observatories and by sun-studying spacecraft can be crucial in alleviating any damage caused by these events.

Even though the sun is – fortunately – relatively stable for now, like all of us it is aging, and in time that will produce changes that will profoundly affect life on Earth.

Via comparative studies between the sun and other stars we have a decent idea of the magnitude and timescales of these changes, although there is still quite a bit left to learn about potential near-term changes in the sun’s behavior.